java 常見問題 之 異常處理不徹底

異常處理不徹底

錯誤的寫法：

try {
 is = new FileInputStream(inFile);
 os = new FileOutputStream(outFile);
} finally {
try {
 is.close();
 os.close();
} catch(IOException e) {
/* we can't do anything */
}
}

is可能close失敗, 導致os沒有close

正確的寫法：

try {
 is = new FileInputStream(inFile);
 os = new FileOutputStream(outFile);
} finally {
try {
 if (is != null) is.close(); } catch(IOException e) {/* we can't do anything */}
try {
  if (os != null) os.close(); } catch(IOException e) {/* we can't do anything */}
}

數據庫訪問也涉及到類似的情況：

Car getCar(DataSource ds, String plate) throws SQLException {
 Car car = null;
 Connection c = null;
 PreparedStatement s = null;
 ResultSet rs = null;
try {
 c = ds.getConnection();
 s = c.prepareStatement("select make, color from cars where plate=?");
 s.setString(1, plate);
 rs = s.executeQuery();
 if (rs.next()) {
 car = new Car();
 car.make = rs.getString(1);
 car.color = rs.getString(2);
}
} finally {
 if (rs != null) try { rs.close(); } catch (SQLException e) { }
 if (s != null) try { s.close(); } catch (SQLException e) { }
 if (c != null) try { c.close(); } catch (SQLException e) { }
}
 return car;
}